The inspiration for micromachining sensors comes from techniques for producing integrated circuits. This consists in producing, collectively on a single thin wafer (in principle a silicon wafer), several tens or hundreds of identical sensors using deposition, doping and photoetching techniques that define not only the electrical parts of the sensor but also the cut-out geometrical features that give the sensor its mechanical properties.
The etching techniques are well controlled and collective fabrication considerably reduces the costs. The robustness of the devices is excellent and the small size of the structures is highly advantageous.
To produce a microgyroscope, a suspended vibrating mass is formed in a silicon wafer together with an electrical excitation structure for making this mass vibrate in a defined direction. When the gyroscope rotates about an axis called the sensitive axis of the gyroscope, perpendicular to this vibration direction, a Coriolis force is exerted on the mass. This Coriolis force, which is a vector sum of the vibration movement and the rotation movement, produces a vibration of the mass in a direction perpendicular both to the excitation vibration and to the axis of rotation. This resulting natural vibration is detected by a detection structure, which is itself produced by micromachining. Structures having two vibrating masses that are mechanically coupled in the manner of a tuning fork have already been produced. The two masses are coplanar and machined in the same silicon wafer.
In general, the sensitive axis of these gyroscopes lies in the plane of the silicon wafer and the detection structure detects any movement perpendicular to the plane of the masses using electrodes placed above each moving mass. The electrical signals resulting from this detection are used to determine an angular velocity of rotation of the gyroscope about its sensitive axis.
However, to produce structures for detecting movements perpendicular to the plane of the moving masses generally requires the gyroscope to comprise several machined wafers, which have to be joined together. One of the wafers includes the actual micromachined vibrating structure with its moving masses, its linking arms and a vibration excitation structure, while at least one other wafer includes electrodes for detecting the vibration generated by the Coriolis force. To fabricate the multi-wafer assembly is expensive.
This is why there is also a need to produce technologically simpler structures, machined in a single silicon wafer, in which an excitation movement of the moving mass is generated in a direction Ox in the plane, whereas a movement resulting from the Coriolis force is detected in a direction Oy in the same plane, perpendicular to Ox. The sensitive axis of the microgyroscope is in this case an Oz axis perpendicular to the plane of the silicon wafer. The excitation structure and the detection structure are interdigitated capacitive combs produced when machining the silicon wafer. All the electrical structures are produced on the same wafer as the vibrating mechanical structure. Fabrication is therefore much less expensive.
In this type of gyroscope it is necessary for the excitation movement along the Ox axis to be well separated from the detection movement along the Oy axis—specifically, this means that the detection structure must detect mainly the movement along Oy that results from the Coriolis force, without the measurement being contaminated by parasitic detection of the excitation movement along Ox.
In the case of a gyroscope of the prior art comprising two vibrating masses and detecting in the plane of their movement, a differential effect is employed to overcome non-linearities and to achieve a high sensitivity. This differential effect consists of subtraction of the signals generated by the movement of the masses, the masses vibrating along the same axis but in phase opposition. When the two masses do not have a perfectly identical static capacitance, the difference in capacitance of the two masses is the cause of a drift of the gyroscope, which impairs its proper operation. Moreover, the gyroscopes of the prior art comprise a detection module for measuring the movement of each mass. These detection modules deliver a signal that changes in the same sense, although the movements of the two masses are the reverse of each other, thereby making the gyroscope sensitive to an acceleration, whether dynamic or static, collinear with the axis of movement of the masses.